This invention relates to a structure for power or distribution transformers (hereinafter "power transformers") using a wound magnetic core, and more specifically relates to a novel support structure for reducing the internal stress on the core.
It has been found that the no-load or iron loss of a wound transformer core can be minimized by reducing or eliminating mechanical pressure on the bottom legs of the core loops. One way to remove the mechanical pressure is to support the coil with insulating pressure plates along each side of the core loops. If the heights of the pressure plates is sufficiently larger than the leg build (E.sub.u) of the core, then the pressure plates will totally support the coil which will, in turn, support the weight of the core structure which drapes over the top of the coil.
The use of a two-piece frame construction using top and bottom channels strapped together over the core and coil assembly is generally limited to the smaller ratings, for example 50 kVA and below. This limitation comes about because of (1) short circuit strength, and (2) weight limitations. Thus, there is a limit to the weight that can be safely supported by the banding straps holding top and bottom channels together. Some manufacturers use a greater number and/or stronger bands to increase the weight capability. Others use a U-frame construction method for higher kVA shell-form designs. This construction greatly enhances the short circuit strength and maximum total weight that can be safely handled.
Iron losses, consisting of both true watt loss "TW" and apparent watt loss "AW" (stored in the magnetic field) can be reduced with the use of pressure plates in a U-frame construction as well as with the two-piece frame construction. If a U-frame construction is used on heavier core-coil assemblies, then it may be necessary to increase the area of the coil insulation that the pressure plates bear upon. Otherwise, mechanical stresses encountered during shipping, handling, and operation of the transformer may crush the insulating margin and cause deterioration or failure of the coil's insulating system. Further, when the coil is supported in a U-frame structure, the wound core will drape over the top of coil and cause added pressure on the coil and internal stress in the core, thus risking further damage to the coil and increased core loss.